The role of protein kinases in the lens has been addressed by studying the endogenous proteins that serve as substrate for the endogenous protein kinases. Two intrinsic membrane proteins MP26 and MP19 are phosphorylated by lens cAMP-dependent protein kinase. In the cortex, cytoskeletal proteins, including the intermediate filament protein, vimentin, are phosphorylated. Oxidative changes of proteins are thought to occur with aging and to contribute to the development of light scattering in the lens. In certain types of cataracts, an increase in oxidative modifications was demonstrated. Studies are in progress to determine the nature of the modifications and the mechanisms leading to the changes.